Electric, fuel cell, and hybrid vehicles include electric drive motor(s) that selectively drive wheels of the vehicle. An energy storage device is provided to store energy that is used to power the electric drive motor(s). Energy storage devices such as ultra-capacitors may be used to provide short bursts of power that may be required during vehicle operation. For example, energy stored in an ultra-capacitor may be used for engine start and stop operation, launch assist and/or regenerative braking.
When implementing an ultra-capacitor, it is important to determine the remaining useful life of the ultra-capacitor energy storage device. FIG. 1 shows an exemplary plot of the instantaneous projected life (Y) of an ultra-capacitor cell as a function of the cell voltage (V) for varying cell temperatures (T). The exemplary plot includes Y at temperatures of 15° C., 25° C., 35° C., 45° C., 55° C. and 65° C. These plots can be expressed according to the following relationship:Y(V,T)=10(aT+bV+c)  [1]where a, b, and c are constants. When Y(T,V) is expressed in years, V in volts and T in ° C., a=−0.03333333, b=−3.33333333 and c=10.1666666666 are useful values.
While the above expression is useful in providing a snapshot of projected lifespan for a variety of specific cell conditions, it does not reflect a projected life based on the cumulative effects of history of operation, nor present time varying cell conditions. Further, neither FIG. 1, nor the expression it represents, defines the method for limiting the ultra-capacitor voltage for the purpose of achieving a desired operating lifespan. Thus a need continues to exist for an improved means and method for optimizing the useful voltage range of an ultra-capacitor power source while achieving a desired operating lifespan over varying in-use operating intervals, especially as an energy storage device for vehicles.